92 PETER GUTHRIE TAIT 



first a thin piece of quartz and then a much thicker piece, he showed how the 

 wave length of the light examined could be determined. There were how- 

 ever several practical difficulties in the way, and the method on trial did not 

 turn out to be so sensitive as Tait had expected. 



The following brief note to Thomson shows that Tait was thinking over 

 the still debated question of the relative motion of the earth and the aether. 



THE CLUB HOUSE, 

 ST ANDREWS, 



26/4/82. 



D' T. 



Srcuf says No ! He says that in such a case period is everything. 

 But I have set Piazzi 1 on to try his magnificent Gitter. If the ether be in 

 motion relatively to the earth, the absolute deviations of lines in the diffraction 

 spectrum should be different in different azimuths : unless (of course) the relative 

 motion of earth and ether be vertical. Anser. Yrs. 



I am not aware however that either Piazzi Smyth or Tait ever tried 

 experiments of the kind indicated. Tait had clearly taken Stokes' opinion, 

 but was not convinced. 



Another problem to which Tait again and again recurred was the question 

 of the diathermancy of water vapour. He strongly doubted the accuracy of 

 Tyndall's well-known experiments on this subject see, for example, his letter 

 to Andrews given above, p. 68. In 1882 he described in a letter to Thomson, 

 who communicated it to the B. A. Meeting at Southampton, a new form of 

 apparatus for investigating absorption of radiant heat by gases. The letter 

 was published in Nature, Oct. 26, 1882. (See Set. Pap. Vol. n, p. 71.) 

 The general idea was to measure the absorption by the increase of pressure 

 in the gas due to the heating. The apparatus was simply a double walled 

 cylinder. While cold water was kept circulating in the jacket, steam could be 

 blown into the double top. The changes of pressure in the gaseous contents 

 were measured by a manometer U tube placed at the bottom. Several series 

 of experiments were carried out by J. G. MacGregor and T. Lindsay 

 (Proc. R. S. E. Vol. xn, 1882, p. 24), the conclusion being "that the 

 absorption of air containing 1-3 per cent, of water vapour is between that of 



1 i.e., Piazzi Smyth, Astronomer Royal for Scotland and Professor of Astronomy in 

 Edinburgh University, a well-known worker in spectroscopy. The signature is a compact 

 monogram giving all three initials P. G. T. The phonetic spelling of "answer" is of course 

 intentional, just as in the case of " Stokes " ; these contractions were frequent, especially between 

 Tait and Maxwell. 



